1. Field
Exemplary embodiments of the present invention relate to an image sensor (IS), and more particularly, to a ramp signal generator capable of reducing horizontal and vertical noise and a CMOS image sensor using the same.
2. Description of the Related Art
In CMOS image sensors, ramp noise generated in a ramp signal generator is included in the ramp signal and transferred to the comparator side through an output resistor, resulting in an increase in horizontal noise.
In this regard, WAKABAYASHI of U.S. Patent Publication No. 2013-0193949 proposes modification of the ramp signal generator in order to reduce the ramp noise. WAKABAYASHI does not stop the ramp noise but reduces the amount of ramp noise that is transferred. Therefore, WAKABAYASHI increases complexity of the circuit.
In CMOS image sensors, when the gain is large, a kTC noise component is removed by correlated double sampling (CDS) operations. However, when the gain is small, the kTC noise component remains despite of the CDS, resulting in the generation of a vertical noise.
In this regard, KOSEKI of U.S. Pat. No. 8,358,360) discloses that a random variation component of pixels is transferred during a comparator reset period in order to reduce the vertical noise at low gain. However, according to KOSEKI, different control timing should be selectively applied only at low gain and pixel data may be distorted due to the adjustment of the pixel control timing.
Furthermore, OIKE of U.S. Patent Publication No. 2012-0019697 discloses that intentionally generated noise can be added during a ramping period within a single row time to reduce the vertical noise. However, OIKE increases the total amount of noise by intentionally adding noise. In addition, OIKE further degrades the horizontal noise.